Two fundamental abilities are central to adaptive human functioning and human development: the ability to deploy actions strategically in service of goals and the ability to apprehend the goals of social partners in order to produce appropriate social responses. Recent neuroscientific findings indicate that these foundational capacities are intimately related: A neural network known as the mirror neuron system (MNS) responds both when we perform an action and when we observe someone else perform that action. These findings hold the potential to revolutionize scientific understanding of the ontogeny of goal-directed action and social-cognitive functioning. However, progress in realizing this potential has been held back by the lack of neural measures of MNS activity in infants, a lack of integrated measures of social-cognitive functions in infants, and the need to develop models of MNS dynamics during development. The Program Project proposed here addresses these needs: Project I will develop EEG methods for assessing MNS activity in human infants, children, and adults as well as infant and adult monkeys. These methods will provide a unified methodological core for the remaining projects. Projects II &III will investigate the social-cognitive functions of the MNS in human infants and infant and adult monkeys, integrating measures of social cognitive functioning with the neural measures developed in Project 1. Project IV will develop a neurocomputational model of the developing MNS, integrating data across all projects. Administrative Core A will provide budgetary and logistical support for each project, provide participant recruitment (Project I, II, IV), and support integrative and outreach activities. EEG/Computational Core B will provide facilities and expertise to support the deployment of the methods developed in Project I across the other projects. This large-scale collaboration emerges from ongoing collaborative work across these laboratories that has laid the groundwork and developed initial methods for taking the program to the next level. The products of this work will provide critical new insights into the development of foundational human capacities as well as innovative new tools, approaches and insights for broader developmental, comparative, and clinical research.